A phase quantum bit is essentially an LC resonator and can be formed from an inductor shunting a Josephson junction. The physical Josephson junction consists of two parts, an ideal Josephson junction and a shunting capacitor. The shunting capacitor can be supplied by the capacitance of the Josephson junction and the inductance can be supplied by the parallel combination of the inductor and the dynamic inductance of the junction. A control circuit is employed to write to the phase quantum bit and a readout circuit is employed to read from the phase quantum bit. The coherence time of the quantum bit is determined in part by the quality factor (Q) of the resonant circuit. Since the control and readout circuits are coupled to the resonant circuit, they load the circuit and reduce the quality factor (Q). In addition, any noise present in the control or readout circuits couples directly to the resonant circuit resulting in decoherence.
A well established technique within the quantum computing community for reducing decoherence resulting from dielectric loss associated with Josephson junctions is to replace the internal capacitance of the Josephson junction with an explicit external capacitor. This can be accomplished by substantially increasing the critical current density of the junction and reducing its area to maintain a constant critical current. As a result, the dynamic inductance of the junction remains constant, but the capacitance is reduced substantially. The missing capacitance is made up with an external shunt capacitor. While this technique can improve poor performance caused by lossy junctions, it does nothing to improve poor performance caused by coupling to control and readout circuits.